1. Field of the Invention
The present invention relates to a current collecting structure of a sealed rechargeable battery of high power.
2. Background Art
Various portable electronic apparatuses have been recently developed. As one important key device, a sealed rechargeable battery serving as a driving power supply has been studied. Especially, a light and small battery of high energy density such as a nickel-metal-hydride rechargeable battery or a lithium-ion rechargeable battery is used in various applications, for example from a consumer-oriented apparatus including a portable phone to a driving power source for an electric vehicle or an electric tool. Especially, the lithium-ion rechargeable battery has received attention as the driving power supply, and has been actively studied in order to increase the energy density and power thereof.
A conventional sealed rechargeable battery for an electric tool or the like requiring high power has a structure shown in FIG. 7A. Electrode group 12, formed by winding positive electrode 3 and negative electrode 6 through separator 11, is stored together with an electrolyte (not shown) in closed-end case 13 made of metal.
Positive electrode 3 has collector 2 and active material layer 1 applied thereon as shown in FIG. 7B. Collector 2 has exposed section 7 having no active material layer 1 at its one end in the width direction. Negative electrode 6 similarly has collector 5 and active material layer 4 applied thereon, and collector 5 has exposed section 8 having no active material layer 4 at its one end in the width direction. Positive electrode 3 and negative electrode 6 are arranged so that exposed sections 7 and 8 lie on opposite sides, and are wound through separator 11 to form electrode group 12. At this time, exposed sections 7 and 8 are exposed at upper and lower end surfaces of electrode group 12.
As shown in FIG. 7A, collecting plate 15 is connected to exposed section 7, and collecting plate 16 is connected to exposed section 8. In this state, electrode group 12 is stored together with electrolyte in case 13, and collecting plate 16 is connected to the inner bottom face of case 13, and collecting plate 15 is connected to a terminal of sealing plate 14. This structure is called a tabless structure. In the tabless structure, the current distribution becomes uniform both in positive electrode 3 and negative electrode 6, and the discharge characteristic of the battery improves. Japanese Patent Unexamined Publication No. 2000-323117 discloses the following structure. Exposed sections 7 and 8 are folded from the outer periphery of electrode group 12 toward the winding axis side sequentially to form welding faces of collecting plates 15 and 16, and collecting plates 15 and 16 are welded to the welding faces, respectively. This method ensures the connection between exposed section 7 and collecting plate 15, and exposed section 8 and collecting plate 16, respectively.
However, when exposed sections 7 and 8 are folded from the outer periphery of electrode group 12 toward the winding axis, the diameter of the large-diameter part becomes small in order. Therefore, the folded part gets wrinkled at each winding position, and hence it is difficult to form a flat welding face. When the exposed sections are folded to form welding faces and then pressurized in order to flatten the welding faces, collectors 2 and 5 buckle at positions between active material layers 1 and 4 and the folded parts. Therefore, the welding faces are not sufficiently flattened and are not sufficiently connected to collecting plates 15 and 16, and hence a target high rate discharge characteristic cannot always be obtained. The buckling may cause distortion in active material layers 1 and 4 to peel them away from collectors 2 and 5, or may partly break active material layers 1 and 4 to disable use of them for charge and discharge. Thus, the battery characteristics including the battery capacity may degrade.